Method of manufacturing an electrochemical cell

ABSTRACT

In order to improve the electrical contact between electrode (1) and current collector (13) in a fuel cell containing a liquid carbonate (10) as the electrolyte it is proposed that between the electrode and the current collector a functional layer (12) be placed. The electrode (11) consists of a material based on lithium cobaltate, and the functional layer (12) is attached thereto. This functional layer, like the electrode, is preferably applied in the green state, and after assembly of the cell, the ultimate electrochemical fuel cell is obtained by sintering.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing anelectrochemical cell having an electrolyte based on molten carbonate.

BACKGROUND OF THE INVENTION

Such a method is known from the German Offenlegungsschrift 40 30 904.There, nickel oxide is used in the electrodes. Between the electrodesand the current collector, a so-called functional layer is employedwhich improves the contact between electrode and current collector. Inthe this Offenlegungsschrift it is proposed to place a "green" foilbetween the electrode and the current collector and then to bake outthis material.

The use of nickel oxide in the electrodes in the first instance providesan effective electrochemical cell, more in particular a fuel cell, butin the long term it was found that internal short circuits arise.Consequently, the use of nickel oxide electrodes is undesirable.

It has been proposed, in the prior art, to employ lithium cobaltateinstead. It was found that this presents greater problems than in thecase of an electrode based on nickel oxide, which relate to thetransition resistance between electrode and current collector.

If, however, a functional layer is used such as that proposed in theabove-mentioned German Offenlegungsschrift 40 30 904, it was found that,as the cell warms up, diffusion of carbonate material takes placebetween the electrode, which for example is in the green state, and thefunctional layer, as a result of which the effect of the functionallayer is lost and the transition resistance increases.

SUMMARY OF THE INVENTION

The object of the present invention is to avoid the above-describeddrawbacks and to provide a fuel cell with electrodes having, on the onehand, an adequate service life and, on the other hand, a low transitionresistance between electrode and current collector by virtue of aneffective functional layer being employed.

This object is accomplished for an above-described method by employing aLiCoO₂ -based electrode material, and attaching the functional layer andthe electrode to each other before placement in a cell stack.

It was found that the use of green lithium cobaltate in conjunction witha green functional layer is possible, if said functional layer isattached to the electrode before this assembly is combined into a cell.

This attachment operation can take place in any manner known in theprior art.

If the tape casting technique is employed, the electrode can befabricated by this method. Then the functional layer can be applied byspraying, screen printing and the like.

It is also, and preferably, possible to fabricate both the functionallayer and the electrode by a tape casting technique and combine themimmediately thereafter. The solvent employed for the binder of both theelectrode and the functional layer preferably has the same basis.

By applying a transition layer or functional layer it is possible toreduce the electrical resistance between the electrode and the metal ofthe current collector. By virtue of this layer being applied in adeformable state to the current collector it is possible to absorbsurface unevenness on the current collector. The same applies tounevenness on the side of the electrode. In the case of use inconjunction with gas ducts, the gas distribution underneath the currentcollection points can be improved. When the fuel cell is warmed up (forexample in the case of fuel cells being stacked) the separate componentsto be assembled consist, according to the invention, of materials of asimilar type. This means that during start-up and operation onlymetal-metal or, for example, oxide-oxide contacts are formed andpreferably no metal-oxide contacts. It is moreover important for theinvention that prior to, during and after these contacts being formed,corrosion problems do not arise, particularly at the interface of the(stainless steel) current collector and the electrode. Corrosion worsensthe electrical contact.

It has been found in certain cases that, after sintering, the thinlayer, on the one hand, is absorbed (in part) by the current collectorand, on the other hand, (in part) by the electrode, so that it can nolonger be distinguished in a sharply defined manner. The above-describedobjectives are achieved, however. The layer being (in part) absorbed bythe current collector indicates that contact formation is not adverselyaffected by (premature) corrosion. The thin, comparatively soft layercan be applied to the electrode or to the current collector (oroptionally corrugation plate) in any way known in the prior art. In thefirst case, when being applied to the electrode, the layer willpreferably contain metal particles for the subsequent formation, duringsintering, of a contact with the current collector. In the second case,when being applied to the current collector or corrugation plate, thelayer will preferably contain electrode material or similar material(oxide) for the contact to be formed subsequently with the electrode.One possible way of applying it is by means of tape casting, the samemethod which is used for the fabrication of electrodes. A correspondingalternative possibility is screen printing. It is also possible tofabricate the layer at the same time as the electrode and to positionthe green assembly thus formed on the current collector. Sintering canthen take place. In the above-described manner, sintering can beaccomplished at the same time as the fuel cell being started up for thefirst time. Thus it is possible for the cell during start-up to beheated for several hours to approximately 500° C., in order to bake outthe necessarily present binder and to melt the electrolyte. Then thecell can be heated to 650° C. This applies particularly to amolten-carbonate cell.

It was found, in particular in the case of the molten-carbonate fuelcell, that, if a stainless steel current collector and a thinnickel-containing layer according to the invention, applied to theelectrode (cathode), are employed, the nickel material diffuses into thestainless steel and thus provides adhesion between current collector andelectrode. Owing to the high nickel percentage, the oxidation resistanceof the current collector increases. Surprisingly it was found that thenickel material does not diffuse into the adjacent electrode or cathode,respectively, so that the properties thereof are unchanged.

A corresponding effect has been observed when cobalt was used as themetal in the interlayer.

The thickness of the functional layer can be between 20 μm and 150 μm.The thickness of the electrode is in the order of magnitude of tenths ofmillimeters.

The invention also relates to an electrochemical cell comprising greencomponents.

The metal employed in the functional layer preferably comprises a metalfrom the iron group.

As stated hereinabove, it is often difficult to distinguish clearly thepresence of the layer according to the invention in the cell inoperation.

The invention will be explained below in more detail in the case of amolten-carbonate fuel cell being employed and more in particular withreference to the interface between cathode and current collector.

In the molten-carbonate fuel cell in question, lithium cobaltate(LiCoO₂) was employed as the cathode material. This material hasadvantages with respect to a cell which is provided with the morecustomary nickel oxide (NiO) cathode. This relates especially to theservice life. If the lithium cobaltate is moreover applied as a greentape, handling is easier and a reduction in cost is achieved, comparedwith a component sintered beforehand. According to the invention it isproposed to provide this green tape with a thin tape which containsmetal powder, the transition resistance between the cathode/currentcollector being considerably reduced as a result.

The above-described lithium cobaltate is preferably employed as apowder. This can be obtained in any way known in the prior art. In thepresent example, the powder was obtained via a solid-state reactionbetween cobalt powder and lithium carbonate. Then, by means of the tapecasting method (doctor blade), a lithium cobaltate tape is produced.During said process a suspension is made from the powder, which consistsof solvent, binder, dispersant, surfactants, anti-foaming agents andoptionally other additives such as pore-forming materials. It does notmatter in this context whether the solvent is organic or water-based.The tapes produced in this manner can be cut to size immediately afterdrying and can be used in a molten-carbonate fuel cell. Any remainderscan, after baking and grinding, be used for a subsequent suspensionpreparation.

Onto the tapes thus obtained, according to the invention a second tapeis cast comprising a metal powder. This tape can be thinner than theabove-described tape and for example has a thickness between 20 and 150μm. It is also possible to cast both tapes simultaneously in oneprocess. The best results are achieved by employing a metal which alsooccurs in the current collector and does not or virtually does not reactto give a lithium-M(metal)-O compound. If the current collector is madeof stainless steel and the tape of lithium cobaltate, as specifiedabove, the best results are obtained with nickel or cobalt.

Moreover it is found that the stainless steel current collector nolonger has to be clad with cobalt, which results in a considerable costsaving.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be illustrated hereinafter with reference to oneembodiment,

FIG. 1 showing the construction of a molten-carbonate fuel cellaccording to the invention,

FIGS. 2a and 2b showing a first way of double tape casting, and

FIG. 3 showing a second way of double tape casting.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a portion of a molten-carbonate cell in a cross-sectionalview.

There, 10 represents the electrolyte in the Li-Al-O₂ matrix, 11 theLiCoO₂ cathode tape, 12 the tape interlayer, 13 the stainless steelcurrent collector, and 14 the stainless steel corrugation plate.

5 indicates gas ducts.

FIG. 2 shows a first embodiment for employing double tape casting. Thisinvolves, as shown in FIG. 2a, applying a layer 21 of tape material to aflat support 22. The tape material may comprise LiCoO₂. Then a broadknife 23 is employed to ensure the homogeneous thickness of said tapelayer. In so doing, the broad knife can either be moved across the layerof tape material 21, or the layer of tape material 21 can be moved pastthe knife together with the underlying supporting surface 22.

The material from which an LiCoO₂ tape is cast is prepared in thefollowing manner:

LiCoO₂ powder, ethanol and tween (surfactant) are mixed together. Thenmethyl cellulose (binder material), demineralized water and TBP(solvent) are added, with continuous stirring.

Then, with the aid of the construction shown in FIG. 2a, a tape is cast.

In a manner not shown a drying cap is positioned over the cast tape inorder to accomplish drying under optimal conditions. After this thickertape obtained, i.e. the electrode material proper, has been dried, athinner second tape 25 is applied in the same manner, as shown in FIG.2b. In so doing, a narrow knife 24 is used.

The nickel suspension employed for tape casting in FIG. 2b is preparedby mixing nickel powder, methyl cellulose, tween, TBP and ethanol. Aftersome time, demineralized water is added and the mixture is stirred forhalf an hour.

Then the functional nickel layer is cast.

FIG. 3 shows a second variation of double casting of a tape, such adouble tape being produced in one step. An LiCoO₂ suspension is preparedin the above-described manner, as is a nickel suspension.

In the process, the nickel-containing layer 25 is applied to the support22, and then the electrode material 21 is placed on top of this. Theright-hand side of the reservoir which contains LiCoO₂ here serves as a(narrow) knife.

After the double tape has been produced, it is placed under a cap, inthe above-described manner, and is then dried.

Adhesion between the functional layer (or Ni layer) and the electrodelayer (LiCoO₂ layer) obtained by means of the above method is so goodthat if the double tape is, for example, rolled up and unrolled again,the layers do not come apart. This is partly caused by the twomaterials, during casting, mixing to a small extent on the boundarylayer and diffusing into each other. This is possible, inter alia, dueto an identical solvent being used.

The following experiment was carried out with the cell shown in FIG. 2:

EXAMPLE

A 100 cm² cell was assembled using LiCoO₂ tape as the cathode and, ontop of this, a Ni tape according to the invention having a thickness of50 μm, and in addition the customary components for a molten-carbonatefuel cell. The cell gave a good and stable performance, approximately790 mV during the duration of the test of 1000 hours, which is notachieved without the interlayer. When the cell was taken apart it wasfound that during operation a firm contact with the current collectorexisted. No undesirable nickel deposits were found in the electrode orthe electrolyte.

Although the invention has been described hereinabove with reference toa preferred embodiment, it should be understood that numerousmodifications thereof are possible without departing from the scope ofthe present application. The above shows that many variations arepossible for the interlayer, while the method for applying it is notlimited to the above-described. Thus it is possible for a completelyfinished electrode to be provided first and for the interlayer accordingto the invention to be applied in the green state between said electrodeand the current collector or separator plate, respectively, and for allof these to be sintered. The sintering can be accomplished as a separatestep.

What is claimed is:
 1. In a method of fabricating an electrochemicalcell having an electrolyte based on molten metal, which methodcomprises:providing a cell stack; bringing the cell stack to an elevatedtemperature; providing an electrode/current collector assemblycomprising an electrode and a metallic current collector; and placing afunctional layer between the electrode and the metallic currentcollector; the improvement which comprises:utilizing a cobaltate-basedmaterial as the electrode material; and attaching the functional layerand the electrode to each other, prior to placement in the stack. 2.Method according to claim 1, wherein the step of attaching thefunctional layer and the electrode to each other comprises providing oneof the electrode and the functional layer and applying thereunto theother of the functional layer and the electrode.
 3. Method according toclaim 2, wherein the step of providing one of the electrode and thefunctional layer comprises fabricating a part comprised of said one ofthe electrode and the functional layer, then applying the other of thefunctional layer and the electrode to said part.
 4. Method according toclaim 2, further comprising using a binder both during the fabricationof the electrode and the functional layer, and utilizing a solvent forthe binder having the same basis.